The present invention relates to steel compositions and processes for treating such compositions that provide a fine-grained austenite microstructure after carburization, useful in cold-formed components which are destined for automotive and machine structural applications.
Various types of highly stressed components for automotive drive train and machine structural use are manufactured by cold forming and carburizing. Cold forming is utilized to reduce energy and material consumption costs associated with various manufacturing processes while case carburizing provides a resultant microstructure with a hard, wear-resistant outer case and a tough, ductile inner core. Cold forced automobile parts such as constant velocity, gears and piston pins are some examples of the end uses of such components. However, cold-formed components are particularly susceptible to abnormal grain coarsening during carburization, and the formation of duplexed grain structures is manifested as problems with distortion, low fatigue life and poor toughness in-the final component. There have been prior attempts to circumvent these problems by utilizing steel compositions and processes that maintain a fine-grained austenite microstructure during carburizing; however, many of these steels and processes are of limited utility in components that experience large reductions and the non-uniform accumulation of strain through the component cross-section during cold forging.
A recent attempt to solve the grain coarsening problem in cold-formed and carburized components is addressed in U.S. Pat. No. 4,634,573 to Yanagiya et al. The Yanagiya et al. patent is directed to a high-nitrogen steel containing grain-refining elements of niobium and aluminum as well as a method of processing that is said to maximize cold formability.
The recent work of Ohshiro et al., published in: Fundamentals of Microalloving Forging Steels, The Metallurgical Society Warrendale. Pennsylvania 1987 at pages, 315-322, addresses the problem of grain coarsening in cold-formed 0.17% C steels with nitrogen contents representative of electric-furnace steelmaking practices (70-130 ppm). Billets of the steels were reheated at temperatures in the 970-1150.degree. C. range and hot roiled to 15 mm diameter rods with a finish rolling temperature of 900.degree. C. (.+-.25.degree. C.). Sections of the wire rods were annealed at 740.degree. C. for three hours, machined into test pieces, cold rolled, reheated at temperatures in the 900-975.degree. C. range for three hours, and water quenched. The Ohshiro et al. work indicates that increases in the dissolution of coarse precipitates with increases in reheating temperature provide an increased amount of solute for the precipitation of fine particles during hot rolling and subsequent annealing at 740.degree. C. Ohshiro et al. also provides data which suggest that heterogeneity in the as-transformed austenite grain size is linked to heterogeneity in the recrystaijized ferrite grain size after cold forming, and this non-uniformity in the as-transformed austenite grain structure is related to the degradation in the grain coarsening resistance of the austenite microstructure during carburizing. Although these data provide an adequate explanation for the formation of duplex austenite grain structures after light (10-20%) cold reductions, these investigators did not indicate any method(s) of controlling this phenomenon. Moreover, the compositions utilized by these investigators are, generally speaking, too lean to provide good grain coarsening resistance under more severe conditions (i.e., high cold reductions followed by carburizing for much longer times).